2 BULLETIN 1449, U. S. DEPARTMENT OF AGRICULTURE 
or less economic importance to the livestock owner. Their presence 
in the meat-producing animals is of even greater significance to the 
veterinary inspector in the abattoir, who passes on the wholesomeness 
of about two-thirds of the meat products consumed in the United 
States. The inspector on the killing floor observes the animal after 
slaughter and consequently renders his decision from the macroscopic 
or gross appearance of the lesions. A definite decision can not always 
be made as to the particular type of tumor found on post-mortem in- 
spection, and consequently many tumors of various types are for- 
warded to the laboratory for a definite diagnosis. This bulletin 
discusses tumors of domestic animals chiefly from the standpoint of 
macroscopic examination of the structure. 
• The word " tumor " means a swelling, but every swelling is not a 
tumor. Inflammation processes, infections by microorganisms, and 
other agencies may produce swellings which are not true tumors. 
It is often difficult to draw a sharp line of distinction between some 
inflammatory swellings and tumors. A swelling produced by in- 
flammation persists as long as the cause producing it lasts and dis- 
appears when the cause subsides. A tumor when once started to de- 
velop continues to grow independently of the surrounding tissue of 
its host. Tumors continue to increase in size even when the host 
becomes emaciated. Age and sex are no barriers to the growth of 
tumors. This independence of tumors is designated by some writers 
as lawlessness or anarchy of growth and is the result of the increase 
in the number of its component elements ; hence the names " new 
growth," " new formation," " neoplasm," and " pseudoplasm " are 
used. These terms mean the multiplication and growth of certain 
cells and serve to differentiate a true tumor from an ordinary swell- 
ing. The more definite term " neoplasm " will be used extensively 
in this bulletin in place of tumor. 
A neoplasm may remain at its original site, the starting point of 
development, and form a primary growth. Later small particles of 
the neoplasm may become detached, enter the blood stream or the 
lymph stream, and be carried to different parts of the body until 
finally lodged, when they form secondary or daughter neoplasms. 
This transmission of the detached particles of the primary neoplasm 
to new loc tions is known as metastasis. This constitutes the law- 
lessness of neoplasms, as the secondary metastatic growths, having 
migrated to a new locality, start an independent existence by form- 
ing secondary neoplasms identical in structure with the primary 
tumor. Primary as well as secondary neoplasms may interfere with 
the function of an organ by mechanical compression of the blood 
supply. For example, when a neoplasm is situated in the cranial 
cavity or in the spinal canal, pressure lesions may eventually cause 
paralysis. 
ETIOLOGY 
The cause of neoplasms is still unknown, notwithstanding the vast 
amount of literature on the subject and all the research done on neo- 
plasms, especially cancers. Various theories have been suggested, 
but they all remain hypotheses that account for individual neoplasms 
only, failing, however, to explain the causation of all neoplasms. 
The recognized theories of the etiology of neoplasm formation are 
stated in the paragraphs that follow. 
